1. Field of the Invention
The present invention relates to laser radar systems and more particularly to a semiconductor laser radar system which utilizes pulse compression techniques by respectively expanding and compressing the transmitted and received laser signals thereby extending the range performance of the semiconductor laser radar system.
2. Description of the Prior Art
Various methods are utilized for measuring the range of a target from a ranging system. For example, radio frequency (RF) radar ranging systems may be utilized. Radar ranging systems are of two general types: (1) continuous wave (CW); and (2) pulsed. A CW radar transmits a continuously modulated waveform and simultaneously listens for the reflected echoes. A pulsed radar transmits its radio waves in short pulses and listens for echoes in periods between transmissions. Each is relatively simple and can be extremely accurate.
For most long-range applications, radar of the pulsed-type is utilized. The principal reason is that with pulsed operation, one avoids the problem of the transmitter, via electrical noise, interfering with the receiver, which in turn limits range performance. Solutions to this problem, i.e., physical separation of the transmitter and receiver, are inappropriate because of space limitations in many systems, such as airborne radar. In addition, pulsed operation has a further advantage of simplifying the range measurement and receiver. If the transmit and echo pulses are adequately separated, a target's range can be precisely determined merely by measuring the elapsed time between the transmission of a pulse and the reception of the echo of that pulse.
In order to obtain both long detection range and fine range resolution, the transmission of extremely narrow pulses of exceptionally high peak power are required. But there are practical limits on the level of peak power one can use.
Pulse compression has been used in radar systems as a solution to this dilemma. In pulse compression, internally modulated pulses of sufficient width to provide the necessary average power at a reasonable level of peak power are "expanded" by modulation and then are transmitted. The received echoes are "compressed" by filters having the conjugate transfer function of the modulation.
The two most common methods of modulation are linear frequency modulation and binary phase modulation. Other methods include polyphase modulation and pulse code modulation.
Because of its parallel to the chirping of a bird, the linear frequency modulation method of coding was called "chirp" by its inventors. With chirp, the radio frequency of each transmitted pulse is increased at a constant rate throughout its length. The received echoes are passed through a filter which introduces a time lag that decreases linearly at exactly the opposite rate at which the transmitted pulses were modulated. Because the successive portions of the received echo tend to "bunch up" after passing through the filter, the filter "compresses" the pulse.
In binary phase modulation and polyphase modulation, as the names imply, the radio frequency phase of the transmitted pulses is modulated. In binary phase modulation, 0.degree. and 180.degree. increments are used, while in polyphase modulation, any number of different, harmonically related phases may be used.
With phase modulation, each transmitted pulse is, in effect, marked off into narrow segments of equal length. The radio frequency phase of certain segments is shifted by the selected increment, e.g., 180.degree., according to a predetermined code. The received echoes are passed through a delay line which provides a time delay exactly equal to the duration of the uncompressed pulses. Thus, as the trailing edge of echo enters the line, the leading edge emerges from the other end, thereby compressing the pulse.
In pulse code modulation (PCM), binary coding information, having optimal correlation properties, e.g., a Barker code, is transmitted within each pulse. The return pulses are cross correlated with the transmitted pulses, that is, the time integral of the product of the two signals, with one having a variable time delay, is taken, thereby resulting in compression of the pulse.
The use of the pulse compression technique simultaneously allows for both long detection range and fine range resolution without exceeding practical limits on the transmitter peak and average powers.
RF radar ranging systems are limited in detectable target size by the size of the RF radar beam used. Thus, RF radar ranging systems are generally not practicable for obstacle avoidance and aim point selection applications where the targets are of relatively small size and the antenna required for the detection of such small targets would be unreasonably large, particularly when such radar systems are used in an airborne application.
In order to detect relatively small targets, laser radar systems have been used because lasers may provide a sufficiently small beam having controlled divergence suitable for detecting these relatively small targets. Laser radar systems, like microwave radar systems, are of two general types: (1) CW; and (2) pulsed. An example of a CW laser radar system which is frequency-modulated is that illustrated by U.S. Pat. No. 4,721,385, issued Jan. 26, 1988, to Jelalian et al. As with conventional radar systems, pulsed laser radar systems are utilized for most applications.
Prior radar systems, whether RF or optical, have used the notion of coherent or heterodyned detection. In particular, RF radars exclusively use coherent detection. In coherent detection, the return echo is electronically mixed with a portion of a separate transmitter signal. The mixed signal provides a beat signal which, when demodulated, contains the target information.
Optical radars using pulse compression have, in the past, been limited to coherent detection to maximize range performance. Typically, utilizing coherent detection requires ultra frequency stable transmitters and high precision receiver detectors. Even with these high quality components, the coherent detection requires receiver video bandwidths which are very large fractions of the carrier. Since laser radars use optical frequency carriers at 10.sup.14 Hz, the receiver bandwidth is quite large, thereby increasing receiver noise and decreasing range performance. In the present invention we utilize the unique approach of using a low frequency carrier, i.e., approximately 100 MHz to impose the encoded or chirped signal. This allows operation of the receiver at lower frequencies where the system noise is less severe and thereby increases the range performance.
In present pulsed laser radar systems, solid state and molecular gas lasers are used to generate the laser beam for tactical range finding applications. These devices, however, are bulky and require large cooling systems which increase system weight, which is undesirable, especially in airborne systems.
At a fraction of the weight and cost of these devices, the semiconductor laser diode is clearly superior. Semiconductor laser diodes, however, are inherently peak power and duty cycle limited due to the thermal recovery required by the active volume of the semiconductor laser. This, in turn, limits the average output power of the transmitter. The present use of a pulsed semiconductor laser radar is inherently limited in its detection range due to the thermal recovery required by the laser transmitter.
For example, present pulsed time-of-flight semiconductor radar systems utilize the pulse delay ranging technique for range measurement. By far, this is the most widely used method of range measurement because it is simple and can be extremely accurate. Basically, when the laser radar's transmission is pulsed, the range of a target can be directly determined by measuring the time between the transmission of each pulse and reception of the echo from the target. The round-trip transit time is divided by two to obtain the time the pulse took to reach the target. This time, multiplied by the speed of light, is the target's range. Using this type of system, if both long detection range and fine range resolution are desired, extremely narrow pulses of exceptionally high peak power must be transmitted. But, due to the inherent physical limits of the semiconductor laser, there are practical limits on the level of peak power available. Thus, the pulsed time-of-flight semiconductor laser radar systems are inherently limited in detection range and range resolution.